IntroductionIt is well-known that in alkali halides doped by alkaline-earth ions or rare-earth ions, electrons cannot be trapped and Z centers are formed due to their relatively low second ionization potentials ( < 15.0 eV) as the first approximation. On the contrary, for some transition metal ions like Mn2+, Z centers cannot be formed because of higher second ionization potentials (> 15.60 eV). The second ionization potential of Mg2+ (15.04 eV) lies just between the above-mentioned two categories of divalent ions [l to 31. At present there are two basic models of impurity centers for MgZf-doped alkali halide crystals: 1. the Z center model i.e., an F center in the neighborhood of a divalent cation [2] to [4] and 2. impurity ions with localized electrons (Mg' and Mg' centers) [5]. Bosi and Nimis [2, 31 discussed the formation of Z, centers in alkali halides adopting a model based on the Born-Haber cycle, and showed that the divalent impurity ions with ionization potentials less than 15.5 eV cannot be reduced to monovalent ions in NaF crystals. So Mg2+ should form Z-type centers rather than be reduced from divalent to monovalent. But they found that the perturbative effects on the F centers in Mg2+-doped NaF crystal is much like FA centers in KCI crystal and mentioned that it may be due to the presence of impurities, such as OH-. They concluded: The F centers in NaF:Mg2+ crystals are probably located in the proximity of the Mg2+-OH-complexes. But Bosi and Nimis 131 did not give exact information about the contents of impurities, especially the behavior under electron irradiation and at higher temperature. The purpose of this work is to study the spectroscopic nature of F centers produced by electron irradiation and bleached with F band light subsequently. The results for Mg2 +-doped samples bleached with F band light
